Silicate and carbonate weathering in the drainage basins of the Ganga-Ghaghara-Indus head waters: Contributions to major ion and Sr isotope geochemistry |
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| Authors: | S Krishnaswami Sunil K Singh |
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| Institution: | (1) Physical Research Laboratory, 380009 Navrangpura, Ahmedabad |
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| Abstract: | The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters
of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are
determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers
flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided
the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ∼ 77%
(Na + K) and ∼ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ∼40%
and in most cases it exceeds the carbonate Sr. The observations that (i) the87Sr/86Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation
between87Sr/86Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly
radiogenic Sr isotope composition of these source waters. The generally low87Sr/86Sr (< 0.720) and Sr/Ca (∼ 0.2 nM/ μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and87Sr/86Sr in the headwaters on a basin-wide scale, however, the high87Sr/86Sr (∼ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of87Sr/86Sr and Ca/Sr data of the source waters show that they diverge from a low87Sr/86Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other
possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this
study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated
CO2 consumption rates and their global significance. |
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| Keywords: | Sr isotopes major ions silicate weathering Himalayan rivers |
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